In this investigation several enzymes formed after T4 phage infection are being monitored in vivo. Basically the technique involves the use of 5-H3-labeled uridine and deoxyuridine. By measurement of H3 released into the medium the activities gf dCMP hydroxymethylase and dTMP synthetase can be followed. This technique has shown that these enzymes must first enter into a complex before they are active. We are studying this complex and its role in DNA synthesis. Another study involves the use of infected cells rendered permeable to nucleotides to follow DNA synthesis. The system shows two pathways to DNA. Both form DNA by a replicative mechanism. The preparation is being used to study the replicative complex.